Perlite is a mineral of volcanic origin which generally falls into the rhyolitic class. The unique feature of perlite is that it contains several percent of water of hydration, which, when the perlite is rapidly heated to a temperature on the order of 1600.degree. F. (870.degree. C.), is converted to steam so that the perlite "pops;" i.e., it rapidly expands to a much lower bulk density. The amount of the expansion is usually on the order of 4-20 times the original volume and the final bulk density of the expanded perlite granules will normally be in the range of about 3.5 to 5.0 lbs/ft.sup.3 (0.06 to 0.08 g/cm.sup.3) for use as insulating fillers or about 7 to 15 lbs/ft.sup.3 (0.11 to 0.24 g/cm.sup.3) for plaster aggregate use.
Expanded perlite is commonly formed in an expansion chamber. In conventional practice this chamber comprises a vertical vessel, usually cylindrical, having ports at both ends. (It will be recognized that there are expanders of other than cylindrical configurations and orientations other than vertical. However, since commercial expanders are most commonly vertical, the present invention and the prior art processes will all be described herein in terms of vertical expanders for brevity. The present invention is, however, applicable to all types of expanders and those skilled in the art will be readily able to apply the descriptions herein to expanders other than the vertical type.) At the bottom end of the vertical expander is a burner which creates a high temperature flame and flame zone within the chamber. The burner operates by being fed fuel gas such as propane or natural gas which is mixed with the appropriate amount of air for proper combustion. Much additional air also enters through the bottom port of the expansion chamber, since the bottom port is open to the atmosphere. The raw perlite ore is injected into the vertical expansion chamber at a point intermediate the top and the bottom. The raw ore falls by gravity into the flame zone where it is rapidly heated and popped. The volume expansion and density reduction upon popping is such that the expanded perlite granules are thereafter buoyant enough to be entrained in the stream of combustion gases and excess air. This stream containing the entrained expanded perlite granules is drawn out of the top port of the expansion chamber by conventional means such as large blowers mounted in the exhaust line leading from the top of the expansion chamber. The expanded perlite granules are thereafter separated from the exhaust gas flow by conventional separation means such as cyclones.
In the past, the bottom of the expansion chamber has been open to the atmosphere and large quantities of excess ambient air were therefore drawn into the expansion chamber. This resulted in major inefficiencies in the expansion process and considerable waste of fuel, since the combustion and perlite expansion was conducted in the presence of varying and often greatly excessive amounts of air at ambient temperatures. Recently a process with much improved thermal efficiencies and perlite expansion yields has been developed and is described in copending U.S. patent application Ser. No. 754,385 filed on Dec. 27, 1976, by K. L. Jenkins and assigned to the Johns-Manville Corporation, which application is incorporated herein in its entirety. Briefly, the Jenkins process involves isolating the inlet port of the expansion chamber from the ambient surroundings, preheating the inlet air by contact with the exterior surface of the expansion chamber and then passing only the preheated air in controlled quantities to the inlet port for mixing with the gaseous fuel, thus optimizing the efficiency of the fuel combustion and the perlite expansion. There are also other facets of the Jenkins process which are described in detail in the aforesaid U.S. patent application and which contribute to its marked superiority over other prior art perlite expansion process.
In the past, all perlite expansion processes, including the aforementioned Jenkins process, have found that reasonable operating efficiency could be obtained only with the use of light fuel. By "light fuel" is meant those fuels such as propane, natural gas and the like which are gaseous under ambient conditions as well as light liquid fuels including liquified petroleum gas (LPG). Attempts to use heavier liquid fuels such as distillate fuel oil and the like, even in atomized form, did not prove successful. In part, this can be attributed to the variable conditions of operating efficiency of the prior art systems, due to the uncontrolled amounts of ambient air present, as discussed more fully in the aforementioned patent application on the Jenkins process.
During the recent severe winters there have been many instances of shortages of natural gas and other light fuels. Normally when such shortages occur, fuel supply companies reserve the available light fuels for residential customers, and industrial customers such as perlite expansion plants have to curtail operations severely or even shut down entirely for lack of the light fuels. However, if an efficent process for the use of heavier liquid fuels were available, such heavier liquid fuels could be stockpiled during periods of ready availability and then used to supplement or replace entirely the light fuels when the latter were in short supply or entirely unavailable. Similarly, such heavier liquid fuels could be used to supplement or replace the light fuels when economic considerations made it less expensive to do so. Such an improved process would obviously be highly desirable.